Alkyl phenols are important intermediates in the synthesis of raw materials for the production of fine chemicals. Alkyl phenols may be synthesized by reacting an alkene and a phenol in the presence of acidic catalyst. Alkyl phenols have broad applications in the production of non-ionic surfactants, anti-oxidants, oil additives, phenolic resin, and so on. An important alkyl phenol is p-tert-octyl phenol [PTOP, or 4-(1,1,3,3-tetramethylbutyl) phenol] because of its use in the production of phenolic resins; p-tert-octyl phenolic resin is widely used as rubber tackifier to produce rubber products, such as tires.
As described in CN101161616A, PTOP may be synthesized as follows: (a) Di-isobutylene (DIB) is obtained by the selective polymerization of isobutylene. The main components of di-isobutylene synthesized in this way are 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. (b) DIB and phenol are reacted under acidic conditions to obtain an alkyl phenol with a highly branched alkyl chain.
However, isobutylene has broad applications in, for example, the production of butyl rubber, oil additives, methyl tert-butyl ether (MTBE), isobutyl alcohol, and other chemicals. Therefore, the supply of isobutylene for DIB production is scarce.
In the rubber industry, p-tert-octyl phenolic resin is often used as a tackifying resin, which usually contains certain amount (2-6% w/w) of residual, free PTOP due to synthetic limitations. Because PTOP is a pollutant, reducing the content of free PTOP in the resin is an important industrial goal. Practical experience proves that increasing the aldehyde-phenol ratio during resin production may reduce the content of free PTOP. However, the application of alkyl phenolic resin in the rubber industry dictates that the softening point of the resin be between 85° C. and 105° C., which facilitates the mixing and dispersion of the alkyl phenolic resin in rubber matrix, improving the rubber tackiness. Traditionally, the aldehyde-phenol molar ratio during synthesis of p-tert-octyl phenolic resin is between 0.7 and 0.9. In fact, when this ratio is above 0.8, the resin's softening point is often above 105° C. Hence, by known methods, it is impossible to reduce the residual PTOP content in the resin by increasing the aldehyde-phenol ratio. In addition, the decrease of free phenol content also improves the tackifying effect of the resin.
U.S. Pat. No. 7,772,345B2 describes a means of reducing free alkyl phenol content in alkyl phenolic resin by reacting vinyl compounds and free alkyl phenol. However the dosage of vinyl compounds is relatively large, and oligomers that do not contribute to the tackifying property will be produced at high temperatures.
CN 1863832A mentions the use of fatty acids to plasticize tackifying resin, thereby increasing the aldehyde-phenol ratio during reaction in order to reduce free phenol. But fatty acids are detrimental to rubber tackiness, and fatty acids will react with zinc oxide in the rubber formulation step, influencing the vulcanization process.
Therefore, it is desirable to offer an alkyl phenolic resin that has less than 2% (w/w) of residual phenol and properties (such as tackiness and softening point) suitable for use in rubber compositions.